Field of the Invention
The present invention relates to non-volatile, high density, integrated circuit memory devices, and more particularly to such memory devices based upon phase change materials such as chalcogenides.
References
The following documents are incorporated by reference herein: U.S. Pat. Nos. 5,837,564; 3,530,441; 4,912,066; 4,455,495; 4,719,594; 5,789,758; 6,077,729; 6,153,890; RE37,259; 5,687,112; 5,789,277; 6,185,122 and 6,150,253. PCT publication PCT/US00/33562. Noboru Yamada, “Potential of Ge—Sb—Te Phase-Change Optical Disks for High-Data-Rate Recording”, SPIE v.3109, pp. 28-37 (1997).
Description of Related Art
Chalcogenides have been utilized in the formation of memory cells for integrated circuit memory devices. Representative prior art patents in this field include Reinberg U.S. Pat. No. 5,789,758, Harshfield U.S. Pat. No. 6,077,729, Wolstenholme U.S. Pat. No. 6,153,890, Ovshinsky U.S. Reissue Pat. No. RE37,259 (Reissue of U.S. Pat. No. 5,687,112), Zahorik U.S. Pat. No. 5,789,277, Doan U.S. Pat. No. 6,150,253, and many others.
Chalcogenides are materials that possess more than one solid-state phase, and which can be switched between such phases using the application of heat caused for example by electrical current or optical pulses. Memory cells which include a chalcogenide element typically are arranged in an array which can be addressed using conventional or novel word lines/bit line addressing schemes common in integrated circuit memories. The state of the memory cell is determined by the bulk resistance of the chalcogenide element. Because the different solid-state phases of the chalcogenide have different resistivity, the bulk resistance of the chalcogenide element indicates the amount of the chalcogenide element in a selected phase state.
In an electrically-programmed chalcogenide-based memory, high electrical current is typically required to program the memory. Often at least 1 mA of current is required to program one bit, though the read current requirements are much less stringent. In a standard MOS IC, it is very difficult to provide a source/drain current of more than 1 mA for a minimum size device. In addition, even if such feats were possible, the high power consumed would be very undesirable for many applications.
The problem of applying current at sufficient current densities to cause the phase change in the chalcogenide element also is reflected in the design of the memory cells. Typically, relatively complex structures are utilized to form small pores in the current path that is coupled to the chalcogenide element. Current is concentrated through the small pores to induce a locally high current density in the chalcogenide element. The complex structures utilized to form the pores, and other aspects of chalcogenide based memory cells, have required relatively large cell sizes to implement. Furthermore, complex structures can affect the reliability of the memory devices. Large cell sizes limit the density of the memory device, and increase its cost.
Chalcogenide materials also are widely used in read-write optical disks, in which laser pulses are used to switch between phases and to read the optical properties of the material after the phase change. Such read-write optical disks include such formats as CD-RW (CD-Rewritable), PD (Powerful Optical Disk System), and DVD-RAM (Digital Versatile Disk-RAM). The structure of these devices is simple and the cost of the disk is low, but it is necessary to prepare an optical system (e.g. DVD-RAM player) to read out the data. The optical system is quite large and expensive comparing to the disk. Moreover, it is slow and power consuming.
Accordingly, neither electrically-written and read chalcogenide memories nor optically-written and read chalcogenide memories are optimal. It would be desirable to provide a chalcogenide memory that can be read without the optical system required by optical disks but does not require the large programming current flow through the device as required by chalcogenide memories that are written and read electrically.